The invention relates to a mass flow sensor, in particular an air mass flow sensor which senses an air mass flow in an intake section of an internal combustion engine.
Mass flow sensors are known which have a sensor element whose measuring signal is representative of a mass flow in a main duct, for example in an intake connector of an intake section of an internal combustion engine. Such sensor elements are preferably embodied as temperature-dependent resistors. The required heating power of the sensor element is then a measure of the mass flow which is to be sensed. The measuring signal of such sensor elements is representative of the mass flow in the main duct but it does not take into account the direction of flow of the mass flow which flows past the sensor element.
In certain load states, in particular at specific rotational speeds, pulsations of the air mass flow occur in the intake section of internal combustion engines. The degree of modulation of the airflow in the intake section in the region of the mass flow sensor may be, for example, up to 300%. In this context the ratio of the oscillation amplitude of the mass flow to the average mass flow is considered as the degree of modulation, expressed as a percentage. For example, the mass flow with a degree of modulation of less than 100% flows only along one main direction of flow, that is to say the cylinders of the internal combustion engine, Only the speed of the flow is subject to certain fluctuations. With a degree of modulation of 100%, the mass flow comes to a standstill at its respective lowest speed. However, there is still no reverse of the direction of flow. Given a degree of modulation of greater than 100%, the direction of flow is respectively for a predefined time period, that is to say the mass flow then flows counter to the main direction of flow. If the degree of modulation is significantly above 100% and if the degree of modulation then increases, a positive measuring sensor of the measuring signal of the sensor signal increases strongly. This is due to the fact that the sensor element senses the mass flow both when it flows in the main direction of flow and when it flows back again, and parts of the mass flow are thus sensed twice.
A precise way of sensing the air mass flow which flows in a collector of the intake section of the internal combustion engine or else into the cylinders of the internal combustion engine is, however, a precondition for precisely setting a desired air/fuel ratio in the respective cylinders of the internal combustion engine.